Sexual reproduction of scleractinian corals has been documented for decades in shallow reefs (Baird et al. 2009), yet little is known from mesophotic coral ecosystems (MCEs, between 30 and 150 m depth) in the Paci c Ocean (but see Holstein et al. 2015, 2016 for the Caribbean Sea). Acropora tenella has been previously documented as one of the abundant species from an upper MCE (40 m depth) in northern Okinawa Island (Sinniger et al. 2013). Gametogenesis and fecundity of this species were described from specimens collected at the same location (Prasetia et al. 2016), but spawning was not observed. Here we report for the rst time the spawning of A. tenella collected and maintained in open air running seawater aquaria. Running seawater was interrupted between 18:00 and 09:00. The colonies were monitored twice a day from 08:00 to 09:00 and from 18:00 to 22:00 from May 29th until 8 days after the main shallow Acropora spawning (May 31st 2015), then between 08:00 and 09:00 and between 18:00 and 19:00 until July 24th 2015. We observed that either 1 or 2 colonies of A. tenella released their gametes in one out of ve outdoor aquaria (2 colonies per aquarium) at some point between 19:00 of June 19th and 09:00 of June 20th 2015. Whitish eggs were found floating at the surface with a mean geometric diameter of 521.2±5.7 μm (mean±SE, n=11) (Figs. 1, 2). The size and the absence of particular stress events at the time of the observation suggest that mature eggs were released although white or pale/cream mature oocytes are rare in Acropora (Baird et al. 2000). Although this observation is too limited to draw conclusion on the exact timing of spawning, it does t with the previous study suggesting that this species spawns at a similar period (i.e. within weeks) to shallow Acropora spp. in the region (Prasetia et al. 2016). Moreover, the positive buoyancy of the eggs may suggest that fertilization happens on the surface. However, this would imply considerable challenges for fertilization, as the gametes of neighboring colonies will be dispersed during the slow ascent to the surface. In addition, since A. tenella is a mesophotic specialist (Veron 2000), the oating eggs also raise the question on how planulae can successfully swim down and nd suitable habitat for settlement. Therefore, future research on the fertilization success and larval dispersal of this species is critical in order to understand the reproductive strategy of corals in the poorly understood deep reef habitats.

Mesophotic “mentions”
5 x (total of 433 words)

Classification
* Presents original data
* Focused on 'mesophotic' depth range
* Focused on 'mesophotic coral ecosystem'

Fields
Reproduction

Focusgroups
Scleractinia (Hard Corals)

Locations
Japan - Ryukyu Islands (Okinawa)

Platforms
Aquarium-based